PART III · Room One · Cosmology

The Big Bang & Cosmic Expansion — Every Galaxy
Recedes, and Space Itself Swells

The universe began 13.8 billion years ago from a state that was unimaginably hot and dense everywhere, and it is still expanding today. This is not conjecture — it is the story told by receding galaxies and by the "first light" left over across the entire cosmos.

§1The Fleeing Galaxies

In 1929, Edwin Hubble discovered that the light from distant galaxies was, without exception, shifted toward the red (redshift). Just as with the Doppler effect for sound, the light from a receding source is stretched to longer wavelengths and turns redder. The astonishing part was the regularity: the farther a galaxy is, the faster it recedes. This is not a picture of an explosion somewhere in the universe scattering matter in all directions — it means that space itself is swelling everywhere at once.

In plain words When you bake a raisin loaf, the dough rises and every raisin moves away from every other one. From the vantage point of any raisin, it looks like "everyone else is receding from me." The farther a raisin is, the more dough lies between, so it recedes faster — this is Hubble's law. The universe has no center.

§2Rewinding Time — The Hot Beginning

Run the film of the expanding universe backward and you reach the very first moment, when all of space was compressed to be endlessly hot and dense. About 13.8 billion years ago, the universe was so hot that not even atoms could exist. As it expanded and cooled, quarks bound together into protons and neutrons (within one second), the first atomic nuclei formed (at three minutes), and finally, 380,000 years later, the universe cooled enough for electrons to be captured by nuclei — and light streamed freely for the first time.

§3The First Light, and the Future

That "first light" has stretched along with the universe for 13.8 billion years, and today it has become microwaves filling the whole sky uniformly — the cosmic microwave background (CMB). Discovered by chance in 1964, this faint glow at an absolute temperature of 2.7 K is the strongest evidence for the Big Bang, and its tiny temperature blotches were the seeds that would later grow into galaxies. Meanwhile, a 1998 discovery revealed that the expansion of the universe is not slowing down but accelerating, and the cause — a mysterious dark energy — makes up about 68% of the universe. The fate of the cosmos remains an open question.

Common misconception

The Big Bang was not "an explosion at a single point within empty space." What exploded was space itself, and the Big Bang happened everywhere at once. It is also possible for a distant galaxy to recede faster than light — not because the galaxy races through space, but because the space between us is stretching.

Key points

  • Hubble's law: the farther a galaxy, the faster it recedes (v = H₀d)
  • What expands is not the galaxies but space itself — the universe has no center
  • The universe is about 13.8 billion years old; rewind and you reach a hot, dense beginning
  • Cosmic microwave background (2.7 K): the "first light" from 380,000 years after the Big Bang — decisive evidence
  • The expansion is currently accelerating — dark energy (about 68%) is the presumed cause
EXP.06 — The Expanding Universet = 0
Observe — Whichever galaxy you take as your reference (yellow), every other galaxy appears to recede. The farther one is, the faster it recedes (the longer its velocity vector) — this is Hubble's law. Use "View from Another Galaxy" to confirm that there is no center.
EXP.06b — 13.8 Billion Years of Cosmic History
From the Planck epoch to today, this is how the universe cooled and matter and structure were born one after another. Click a node to see what happened in each era.
COSMIC TIMELINE13.8 billion years
Click a node on the timeline to explore each era
Big Bang (t = 0) — The beginning of time and space. The earliest moment describable by known physical laws is the Planck epoch (10⁻⁴³ seconds).
DEEP DIVE — Equations & History
Hubble–Lemaître LawHubble, 1929
$v = H_0\, d$
v the galaxy's recession velocity · d distance · H₀ the Hubble constant (≈ 70 km/s/Mpc). This simple relation — that velocity is directly proportional to distance — is direct evidence of an expanding universe. The reciprocal 1/H₀ gives a rough age of the universe (about 13.8 billion years).
Redshift · Friedmann EquationThe Dynamics of Expansion
$1+z = \frac{\lambda_{\text{obs}}}{\lambda_{\text{emit}}} \qquad \left(\frac{\dot a}{a}\right)^2 = \frac{8\pi G}{3}\rho - \frac{kc^2}{a^2}$
Left: the redshift z is how much the light has been stretched — it is light that traveled while space expanded by a factor of (1+z). Right, the Friedmann equation applies Einstein's equations to the universe as a whole and determines how a (the size of the universe) changes with the matter density ρ.
HISTORY — Cosmology Timeline
1915
Einstein's field equations → the door opens to physics that treats the universe as a whole
1927
Lemaître proposes the expanding universe and the "primeval atom" hypothesis
1929
Hubble confirms it by observing the galaxy recession-velocity–distance relation
1964
Penzias & Wilson accidentally discover the cosmic microwave background (decisive evidence for the Big Bang)
1998
Supernova observations reveal the accelerating expansion of the universe → dark energy (2011 Nobel Prize)
← PREV
Black Holes